The invention relates generally to nuclear magnetic resonance imaging (“MRI”), and more particularly to a technique for using a combined pulse sequence wherein the imaging and tracking functions share the radiofrequency (“rf”) excitation pulse to allow for simultaneous MR imaging and device tracking.
MRI systems have become ubiquitous in the field of medical diagnostics. In general, MRI systems are based on the interactions among a primary magnetic field, an rf field and time varying magnetic gradient fields with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be influenced by manipulation of the fields to obtain rf signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of three gradient coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. A radiofrequency coil is employed to produce an rf magnetic field, typically pulsed to create the required resonance signals. This rf magnetic field perturbs the spin system from its equilibrium direction, causing the spins to precess at desired phases and frequencies. During this precession, rf fields are emitted by the spins and detected by either the same transmitting rf coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to reconstruct a useful image.
Many specific techniques have been developed to acquire MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and rf pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as “pulse sequences” in which the pulses are represented along with temporal relationships among them.
Heretofore, MRI systems have also been employed for device tracking during medical (e.g., surgical) procedures. MR tracking generally employs small tracking coils attached to the device to be tracked. During these MR tracking procedures, signals are generated throughout the patient using a large transmitting rf coil, but are detected with the small tracking coils attached to the device. In one example, locating the tracking coils may be typically accomplished by acquiring the MR signal in the presence of the applied magnetic field gradient, Fourier transforming the signal, and identifying the position of the most intense frequency-domain signal. In a manner similar to MR imaging, the gradient pulses and rf pulses used to manipulate the spin systems in MR tracking may be graphically represented as pulse sequences.
Because MR tracking utilizes much of the same hardware, instrumentation and physical phenomena as MR imaging, the device location can be overlaid onto an MR image. So that both the device location and the image may be updated during the medical procedure, the MR tracking pulse sequences are typically interleaved with the MR imaging pulse sequences. However, unlike most MR imaging methods, MR tracking can be performed rapidly (e.g., faster than 20 frames per second) over the entire three-dimensional volume of the patient. As such, the device location may be updated more frequently than the MR image. During MR tracking, it is often highly desirable that the real-time representation of the device be visualized with respect to a reference image that accurately represents the patient anatomy. In practice, this may be difficult to achieve because of patient motion and/or changes in anatomy (or function) as a result of the medical procedure.
Accordingly, there is a need for an improved technique for employing device tracking with an MRI system. Particularly, there is a need for a technique that provides for more timely synchronization between the acquisition of tracking data and imaging data.